Consumer groups and the beef industry in the United States were yesterday attempting to avert any public panic over food supplies after officials at the Department of Agriculture revealed on Thursday that they had uncovered a possible case of BSE.

The government admitted that preliminary screening for the disease, which can be transmitted in a usually fatal form to humans, had proved positive on one animal. It insisted the results were considered "inconclusive" pending further tests.

The results may not be ready for several days and in the meantime the country's $40bn- a-year (£21bn) beef industry can only wait anxiously. The news sent beef prices tumbling and cast an instant chill on the main fast-food chains' share prices. While some experts warned it was likely the second round of tests would confirm the presence of BSE (bovine spongiform encephalopathy) officials at the Department of Agriculture were more sanguine.

While refusing to reveal where the animal had been found, Andrea Morgan, associate deputy administrator of the department's inspectors, said the agency "remains confident in the safety of the US beef supply".

When the US found its first confirmed case of the disease in an imported cow last year, scores of countries, including Japan and Taiwan, banned imports of American beef. The bans have only recently been lifted. Exports account for about $3.8bn every year. Even more disastrous than a loss of international trade, however, would be a turn against beef by US consumers.